The present invention relates to a bucket cooled by circulating water and a scrap preheating apparatus incorporating the bucket. In particular, it relates to a scrap preheating apparatus which utilizes as its heat source the exhaust gas from an electric furnace.
Preheating of scrap has recently come to be practiced in the manufacture of steel by electric furnaces since it markedly reduces the unit electric power and also increases productivity. Several systems for preheating scrap have been actually installed in electric furnaces and have proved to be efficient.
Exhaust gas from an electric furnace is utilized to preheat scrap according to two different methods. In one method, the exhaust gas is combusted before being used for preheating. In the other method, the exhaust gas is used for preheating scrap without being combusted or otherwise treated beforehand.
There are two types of apparatus for preheating scrap. In one type, a bucket containing scrap is placed in the preheating apparatus and both the scrap and the bucket are heated. The preheated scrap is then removed from the apparatus together with the bucket. In the other type, the bucket itself is utilized as a preheating apparatus. Namely, an exhaust gas is passed from the top to the bottom of the bucket to preheat the scrap.
Up to the present time, it has been most common to preheat scrap by combusting exhaust gas from an electric furnace and then introducing the combusted gas into a preheating apparatus containing a bucket, thereby preheating not only the scrap contained in the bucket but also the bucket itself. This method is called the indirect preheating method.
However, the indirect method has poor heat efficiency, and equipment costs are high since it requires a combustion apparatus as well as a preheating apparatus. Thus, the indirect preheating method is not economically attractive.
As opposed to the indirect method, there are direct preheating methods in which exhaust gas is directly supplied from an electric furnace to a preheating bucket without being first combusted, and the bucket itself is used as a preheating apparatus. Direct methods have high heat efficiency and do not require a combustion apparatus or an additional preheating apparatus. However, as the temperature of exhaust gas is over 500.degree. C., the exhaust gas sometimes produces serious thermal deformation of the bucket containing the scrap. Thus, it is necessary in a direct method to provide some means for protecting the bucket from excess heat.
In order to avoid therxal deformation of a bucket used in a direct heating method, Japanese Patent Publication No. 1751/1982 suggests the provision of a water jacket around the bucket. A high-temperature exhaust gas from an electric furnace is introduced from the top of the bucket, and the wall of the bucket is cooled with water.
However, according to the system disclosed therein, as the cooling water remains stationary within the water jacket, the cooling effect thereof is not sufficient and it is impossible to continue preheating the scrap until a desirably high temperature (above 500.degree. C.) is attained. Namely, when preheating is continued until such a high temperature is attained, the cooling water in the jacket vaporizes and an extremely high pressure develops within the jacket, and there is the possibility of steam or water leaking from the jacket due to the high pressure. Furthermore, there is the danger of water leaking or overflowing from the jacket during transport of the bucket, and thermal deformation of the bucket is inevitable. A bucket of this type thus has drawbacks with respect to safety.
Furthermore, it is sometimes necessary to keep the preheated scrap within a bucket before charging it into an electric furnace because of some operational troubles. While keeping such preheated scrap within the bucket, water contained in a jacket provided around the bucket evaporates due to the heat of the scrap and the pressure within the jacket increases. A burst of steam out of the jacket might occur through an exhaust port.
In addition, when a cylindrical bucket measuring 4 m in diameter and 4 m in height and weighing 12 tons is carried by means of a crane and is placed at a given site, it is impossible to position the bucket with an accuracy of 20 mm in each direction. Thus, it has been thought impractical to employ a water-circulating cooling system for a bucket because the difficulty in positioning a bucket would make it extremely difficult to reliably connect the bucket to a source of cooling water.